The present invention relates generally to improved methods and apparatus for use actuating an explosive charge downhole in a well bore. More specifically, the present invention relates to methods and apparatus for performing such actuation through use of a time delay apparatus which provides a relatively long duration delay through use of a series of combustible columns which combust in a predetermined, sequential order.
As is well known in the art, downhole explosive devices are utilized downhole in well casing to detonate explosive shaped charges which perforate the well casing and surrounding formation. A downhole explosive device, such as a perforating gun, is typically actuated through use of a firing head which is responsive to either mechanical forces or fluid pressure. So-called mechanically-actuated firing heads are typically responsive to an impact, such as may be provided by the dropping of a detonating bar through the tubing to impact an actuation piston in the firing head.
In many situations, however, mechanically-actuated firing heads do not provide sufficient reliability for use in the wellbore because the wellbore environment includes such interferences to the dropping detonating bar as debris and particles settling out from the heavy drilling muds. Also, it is frequently desirable to monitor various downhole parameters, such as temperature and pressure, through the use of instruments mounted between the tubing string and the firing head. These non-fullbore opening devices typically will not permit a detonating bar to pass through to the firing head. In such applications, pressure responsive, or so-called "hydraulically-actuated", firing heads are utilized.
Hydraulically-actuated firing heads are responsive to a source of fluid pressure, either in the well tubing or the well annulus, which moves an actuation piston in the firing head to initiate detonation of the perforating gun. Such firing heads require a specific source of substantial fluid pressure. However, it is often desirable to perforate the casing underbalanced. In such a situation, a time delay is desired between the time when the actuation signal is received by the firing head (via the hydraulic pressure) and the time when the actuation signal is transmitted from the firing head to the explosive device, detonating the explosive shaped charges and perforating the well casing. During this time delay, the pressure in the casing can be adjusted from the pressure required to actuate the hydraulically-actuated firing head to the pressure desired during the perforation of the well casing.
One conventional method of providing a time delay is to provide the firing head assembly with a relatively slowly combustible column between the firing piston and the explosive charges. One such combustible column which is known to provide such a time delay is a tungsten column. An exemplary time delay firing head utilizing such a tungsten column is disclosed in U.S. Pat. No. 4,614,156, issued Sep. 30, 1986, to Colle, Jr. et al., and assigned to the assignee of the present invention. The disclosure of U.S. Pat. No. 4,614,156 is hereby incorporated herein by reference for all purposes. By adjustment of the length of the column between the firing piston and the explosive charges, the desired time delay can be provided. One advantage of this method is that a time delay of a relatively definite period may be provided.
One side effect of using such a combustible column in the firing head assembly is that the firing head assembly is necessarily lengthened to provide for the elongate combustible column. As a result, the firing head assembly is more difficult to assemble into the tool string on the job site. Additionally, there are practical limitations upon the length of a time delay which is feasible with a single extended delay column.
In addition, due to its length, the longer firing head assembly presents difficulties in manufacturing and shipping. This longer firing head assembly may also present additional problems in some downhole applications where a longer tool string is disadvantageous.
In the prior art, one solution to the problems in manufacturing and shipping is to provide a time delay by longitudinally connecting a series of shorter tungsten delay columns. Such a design is disclosed in U.S. Pat. No. 5,062,485 to Wesson et al. This design is useful in applications requiring a relatively short duration delay. However, in applications requiring a relatively long duration delay, a number of these shorter tungsten delay column units are required. In such applications, this design does not overcome the problems at the job site associated with the assembly of a series of delay column units and the length of the resulting tool string downhole.
In addition, these delay column units, each incorporating a tungsten delay column, are designed to be screwed into a tool string in the field as needed. As a result, a detonating cord cannot be potted in place from one tungsten delay column to the next to communicate the combustive reaction therebetween. Instead, an explosive charge, firing piston and initiator are necessary to communicate the combustive-reaction from one of these delay column units to the next. Because of the requirement of these additional parts in each of the shorter tungsten delay column units, this design may not be economically efficient, from either a manufacture or shipping viewpoint, in applications requiring a relatively long duration delay. Also, as additional explosive charge/firing piston/initiator combinations are used, there is an increased possibility that one of the explosive charge/firing piston/initiator combinations will not fire properly. Finally, the additional delay columns require additional time to be assembled into the tool string in the field.
Accordingly, the present invention provides new methods and apparatus whereby detonation is delayed through use of a single, abbreviated delay apparatus utilizing a plurality of generally coextensive, tungsten delay columns. Because the apparatus is a shorter, solitary unit, it is more easily shipped and is more easily and quickly assembled into a tool string in the field. Since the apparatus is manufactured as a single unit, the ends of a detonating cord can be potted in place in the combustible columns during manufacture. Thus, the combustive reaction may be communicated from one combustible column to the next generally coextensive combustible column through use of a detonating cord, thereby eliminating the need for an explosive charge/firing piston/initiator combination therebetween. Thus, the problems associated with the conventional methods and apparatuses for providing a time delay are avoided.